1. Technical Field
The present invention relates to a backlight unit, and more particularly, to a backlight unit having a polarization layer attached to an upper surface thereof, a method for manufacturing the same, and a liquid crystal display device comprising the same.
2. Discussion of the Related Art
Advances in information technology result in rapidly increasing demands for display devices having various shapes. Correspondingly, various flat panel display devices, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electro luminescent display (ELD) devices, vacuum fluorescent display (VFD) devices, and the like, have been continuously investigated, and some of them have been already applied to various apparatuses in practice.
Among these flat panel display devices, the LCD devices are most widely used for a mobile image display device in place of CRT monitors in view of their merits including excellent image quality, light weight, compactness, and low power consumption. Specifically, the LCD devices are developed for monitors of TV sets which can receive and display broadcasting signals, and monitors of computers in addition to mobile display devices such as notebook computers.
In this regard, although various technological developments have been accomplished to enable the LCD devices to act as a screen display device in various fields, the LCD devices still have many problems in operation for improving the quality of an image as the screen display device, which result in failure to obtain the merits of the LCD device as mentioned above.
Accordingly, in order to allow the LCD devices to be applied to various apparatuses as a general screen display device, it is necessary for the LCD devices to realize high quality images with characteristics such as high definition, high brightness, large size while maintaining the merits such as light weight, compactness, and low power consumption.
A conventional liquid crystal display device will be described as follows.
FIG. 1 is a schematic view illustrating a conventional liquid crystal display device.
Generally, the LCD device comprises a liquid crystal panel to display an image, and a driving unit to drive the liquid crystal panel.
In the conventional LCD device, the liquid crystal panel denoted by reference numeral 1 comprises a liquid crystal cell 11, upper and lower polarization plates 12 and 13, and a backlight unit 10 to illuminate light to the liquid crystal panel 1, as shown in FIG. 1.
Although not shown in the drawings, the liquid crystal cell 11 comprises upper and lower substrates assembled to each other with a predetermined space defined therebetween, a liquid crystal layer formed between the upper and lower substrates, and a spacer to maintain a uniform cell gap in the liquid crystal layer. The upper and lower polarization plates 12 and 13 are coated on outer surfaces of the upper and lower substrates.
The driving unit comprises the backlight unit 10 to uniformly illuminate light to the liquid crystal panel, and driving circuits (not shown) to apply driving signals to the liquid crystal panel.
The backlight unit 10 comprises a lamp 15 provided at one side of a light guide plate 14, a lamp housing 17 surrounding the lamp 15 to protect the lamp 15, the light guide plate 14 to guide light emitted from the lamp 15 to be uniformly transmitted to the liquid crystal panel 1, a light scattering assembly 16 disposed on the light guide plate 14 to allow the light emitted upwardly through the light guide plate 14 to be uniformly transmitted to the liquid crystal panel 1, and a protective sheet 18 disposed on the light scattering assembly 16. The light scattering assembly 16 comprises a plurality of diffusion sheets, and horizontal and vertical prism sheets.
The upper and lower polarization plates 12 and 13 are respectively coated on the outer surfaces of the upper and lower substrates in a direction that transmission axes (absorption axes) thereof cross each other.
Generally, light is an electromagnetic wave, and has a vibration direction perpendicular to a direction of propagation. Polarized light is light biased in the vibration direction of light. That is, the polarized light means light which strongly vibrates in a specific direction among perpendicular directions with respect to the direction of propagation.
A polarization plate splits an incident light into two orthogonal polarization components, and allows only one of them to pass therethrough while absorbing or dispersing the other.
The backlight unit 10 emits light which vibrates with equal probability in all directions. The upper and lower polarization plates 12 and 13 allow only a component of light vibrating in the same direction as that of the polarization axis to be transmitted therethrough while absorbing or reflecting other components of light vibrating in other directions via a suitable medium, thereby providing light which vibrates only in a single specific direction.
Since the upper and lower polarization plates 12 and 13 are attached to upper and lower surfaces of the liquid crystal layer to be orthogonal or parallel to each other, it is possible to exhibit black, white and grey therebetween by regulating the intensity of transmitted light according to a rotating degree of the polarization axis, while light passes through the liquid crystal layer.
As such, the conventional LCD device displays an image by regulating light with the liquid crystal layer having a thickness of several micrometers between the upper and lower substrates, and the upper and lower polarization plates provided on the outer surfaces of the upper and lower substrates. For reference, a reflection type LCD device employs an external light source instead of the backlight unit, and displays an image by regulating light with the liquid crystal layer, a single polarization plate, and a single reflection plate.
In order to regulate light as described above, the liquid crystal display device displays the image by converting non-polarized light entering from the backlight unit or an external light source into polarized light, and controlling the polarized light using properties of the liquid crystal layer to regulate the intensity of emitted light.
For the conventional LCD device constructed as described above, the upper and lower polarization plates 12 and 13 are separately manufactured, and attached to the upper and lower portions of the liquid crystal cell 11. However, the conventional LCD device has problems in that, when attaching the separately manufactured polarization plates, there is possibility of defective assembly, and that it is inconvenient to separately manufacture the polarization plates.